The inventions disclosed herein relate to the cleaning of semiconductor wafers, next generation lithography (NGL) masks, and optical photomasks during their manufacture, and more particularly to the cleaning of semiconductor wafers, NGL masks, and optical photomasks by the use of plasma gases, reactive gases, and mechanical agitation.
Particle contamination on semiconductor substrates can cause defects in finished semiconductor devices resulting in yield loss and/or reduced reliability of semiconductor devices. Accordingly, the semiconductor manufacturing industry has paid considerable attention to maintaining high standards of cleanliness during the manufacture of semiconductor devices, and clean room technology has evolved to a level such that particle deposition onto substrates from exposure to the clean room is only a minor source of substrate contamination. Rather, most of the contamination found on substrates derives from process tools, processing materials, and interior walls of the processing chamber. Accordingly, substrates must be cleaned before, during, and after many of the processes steps they must undergo to become a finished semiconductor device. NGL masks pose an additional need due to the fact that these masks cannot employ the protective pellicle covering which prevents particle contamination on traditional masks. Cleaning accounts for approximately 30 percent of the processing performed on semiconductor fabrication line.
U.S. Pat. No. 5,849,135 to Selwyn, the disclosure of which is herein incorporated by reference, describes a system for particle contamination removal from circular semiconductor wafers using plasmas and mechanical agitation. Generally, the Selwyn Patent describes a method and apparatus in which particulate matter can be removed from the surface of a wafer by forming an RF plasma sheath in the vicinity of the surface of a wafer such that the wafer surface is bombarded by positive ions and electrons from the plasma. Mechanical resonance vibration means are used to introduce vibrations in the wafer perpendicular to its surface thereby breaking the bonds between the particulate matter and the substrate such that particulate matter is caused to move away from the surface of the wafer and into the plasma sheath. Once free of the wafer surface, the particulate matter becomes negatively charged by the electrons in the plasma and are drawn into the plasma by attractive forces which keep them from redepositing. The introduction of a flowing gas through the plasma sweeps the particulate matter away from the wafer and out of the plasma.
More specifically and with reference to FIG. 1 of the Selwyn Patent, the cleaning apparatus depicted therein is comprised of a vacuum chamber which includes a circular shaped RF electrode and a circular shaped ground electrode. The RF electrode is capacitively coupled to an RF power source. A retaining ring is suspended above the wafer to restrict the wafer""s travel. Plasma is formed between the RF electrode and the ground electrode when RF energy is applied to the RF electrode by the RF power source. The plasma sheath is located above the semiconductor wafer and below RF electrode. The semiconductor wafer is caused to vibrate at approximately 10 kHz by means of a conducting post which passes through the walls of the vacuum chamber and which is driven by a mechanical vibrator.
Gas, preferably a noble gas such as helium or neon is introduced into the vacuum chamber via an inlet tube thereby establishing a radial gas flow. A pair of vacuum pumps permit the vacuum chamber to be operated at 1-10 torr while the radial gas flow is generated. Strong drag forces generated by the high gas flow rate drive the particulate matter out of the plasma and into the pumping ports of the chamber.
However, the plasma cleaning process described in the Selwyn Patent does not remove all contaminant particles from a wafer. Experience has shown that the cleaning efficiency of the method described in the Selwyn Patent (i.e., the number of particles on the wafer prior to plasma cleaning less the number of particles on the wafer after the cleaning process divided by the number of particles on the wafer prior to plasma cleaning and multiplied by 100) is approximately 70 percent for W particles at 1.25 micron size. Nowhere does the Selwyn Patent, or any other prior art known to the inventors, teach or suggest the use reactive gases to improve the cleaning efficiency of a plasma cleaning process by breaking the chemical bonds between contaminant matter and a silicon wafer. Indeed, the Selwyn Patent at Col. 6, lines 26-30 explicitly teaches away from the use of reactive gases. Additionally, nowhere in the prior art is it taught or suggested that broadband or impulse vibration means can be used to agitate the substrate.
Accordingly, it is an object of the present invention to provide a reactive plasma cleaning method and apparatus having an increased cleaning efficiency.
Additionally, it is an object of the present invention to provide a plasma cleaning method and apparatus utilizing broadband or impulse vibration means.
Further, it is an object of the present invention to provide a reactive plasma cleaning process that can be used for the cleaning of a variety of devices including NGL masks and optical photomasks as well as semiconductor wafers.
It is further an object of the present invention to provide a reactive plasma cleaning process and apparatus that may be used to clean contaminant particles of various chemical compositions from a variety of substrate compositions.